The present invention relates to wireless telecommunication systems. More particularly, and not by way of limitation, the invention is directed to a method and mobile terminal for providing accurate and timely uplink scheduling information from the mobile terminal to a scheduler in a wireless telecommunication system.
The following acronyms are used in the description herein:
3GPP Third Generation Partnership Project
BS Buffer Size
BSR Buffer Status Reports
CBRA Contention-Based Random Access
CE Control Element
CQI Channel Quality Indicator
C-RNTI Cell-specific Radio Network Temporary Identifier
DRB Data Radio Bearer
D-SR Dedicated resources for Scheduling Request
eNB eNodeB
E-UTRA Evolved UMTS Radio Access
GBR Guaranteed Bit Rate
HOF Handover Failure
LCG Logical Channel Group
LCH Logical Channel
LCID Logical Channel Identifier
LTE Long Term Evolution
MAC Media Access Control
PBR Prioritized Bit Rate
PDCCH Physical Downlink Control Channel
PDU Packet Data Unit
PUCCH Physical Uplink Control Channel
PUSCH Physical Uplink Shared Channel
QoS Quality of Service
RA Random Access
RAR Random Access Response
RA-SR Random Access-based Scheduling Request
RB Resource Block or Radio Bearer
RBG Radio Bearer Group
RLC Radio Link Control
RLF Radio Link Failure
RRC Radio Resource Control
SDU Service Data Unit
SR Scheduling Request
SRB Signaling Radio Bearer
SRS Sounding Reference Signals
TB Transport Block
TTI Transmission Time Interval
UE User Equipment
UL-SCH Uplink Shared Channel
UTRAN UMTS Terrestrial Radio Access Network
In 3GPP, work is ongoing on specifications of the evolved UTRAN (E-UTRA) as part of the Long Term Evolution (LTE) effort. The 3GPP technical specification TS 36.321 v8.3.0 (LTE MAC) describes uplink information for scheduling for LTE systems. The 3GPP technical specification TS 36.331 v8.3.0 (LTE RRC) describes procedures including handover and Radio Resource Connection (RRC) Connection Re-establishment. Both documents are incorporated herein by reference in their entirety.
In LTE, scheduling is modeled in the Medium Access Control (MAC) layer and is performed by a scheduler residing in the eNodeB (eNB). The scheduler assigns radio resources, also called Resource Blocks (RBs), for the downlink (assignments) as well as for the uplink (grants) using the Physical Downlink Control Channel (PDCCH). To assist downlink scheduling decisions in the eNB, the mobile terminal or User Equipment (UE) can be configured to transmit Channel Quality Indicator (CQI) reports on a configured resource (PUCCH) or on a dedicated resource (PUSCH). CQI reports are typically based on Sounding Reference Signals (SRS). The scheduler uses the CQI information to perform fast channel dependent link adaptation and to change allocations in the time and frequency domains.
For uplink scheduling, the eNB needs information about the current state of the buffers in the UE, i.e., how much, if any, data the UE has in its priority queues. Precise and up-to-date scheduling information enables the scheduler to make more accurate scheduling decisions, and can help to optimize the use of radio resources and to improve capacity. The 3GPP technical specification TS 36.321 specifies a framework for the UE to report to the eNB, the amount of data stored in its buffers for transmission. The eNB uses these Buffer Status Reports (BSRs) to allocate resources to the UE, and to prioritize resource allocation between different UEs. The UE triggers a Regular BSR and a Scheduling Request (SR) when uplink data becomes available for transmission and the data either belongs to a radio bearer (logical channel) group with higher priority than those for which data already existed in the buffer, or when the UE buffers were empty just before this new data became available for transmission. With this type of triggering, the scheduler can quickly be made aware when data with higher priority is available for transmission, without excessive reporting.
However, the accuracy of the information provided by the UE is limited by the granularity of the BSRs, by the frequency of the BSR transmissions, and by the delay between the arrival of data and the processing of the BSR received by the eNB. With insufficient or inaccurate uplink information, the scheduler is likely to provide either a grant which is too large (which then results in the UE transmitting padding and may reduce system capacity) or a grant which is too small (which may lead to RLC segmentation and increased transmission delay).
Uplink BSRs are transmitted using MAC signaling. As noted in 3GPP TS 36.300, uplink BSRs are needed to provide support for QoS-aware packet scheduling. In E-UTRA, uplink BSRs refer to the data that is buffered for a logical channel group (LCG) in the UE. Four LCGs and two formats are used for reporting in the uplink:                a short format for which only one BSR (of one LCG) is reported; and        a long format for which all four BSRs (of all four LCGs) are reported.        
When the UE receives a grant for a new transmission on the Uplink Shared Channel (UL-SCH), the decision of how much data to transmit from each logical channel, i.e. the uplink prioritization, is performed in two rounds. In the first round, logical channels are served in a decreasing priority order up to their configured Prioritized Bit Rate (PBR) or until the data available for transmission for that logical channel is exhausted. Note that both Guaranteed Bit Rate (GBR) and non-GBR bearers may be allocated a PBR. In the second round, the logical channels are served in priority order until the data available for transmission for that logical channel is exhausted or until the received grant is exhausted. This provides a way to solve starvation issues because eNBs can configure the PBR of the higher priority logical channel in such a way that not all the UL resources are taken.
Whenever the transport block size is larger than the amount of data available for transmission at the time of assembly of the MAC PDU for transmission, a BSR known as a padding BSR can also be included. In the padding BSR case, a truncated format is also available whenever the terminal has data for more than one logical channel group but the
MAC PDU only has room for a BSR format that is restricted to information about one logical channel group. Another type of BSR defined in TS 36.321, the Periodic BSR, provides a timer-based trigger per UE to handle reporting for continuous flows.
The BSR is defined as a MAC Control Element (CE). The BSR CE consists of either a short or truncated BSR format containing one Logical Channel Group (LCG) ID field and one corresponding Buffer Size (BS) field, or a long BSR format containing four BS fields corresponding to LCG IDs #1 through #4.
FIG. 1A is an illustrative drawing of the short or truncated BSR format 10.
FIG. 1B is an illustrative drawing of the long BSR format 15.
FIG. 2 is an illustrative drawing of an existing MAC Packet Data Unit (PDU) 20 as defined in the 3GPP technical specification TS 36.321. The MAC PDU consists of a MAC header 21, zero or more MAC Service Data Units (MAC SDUs) 22, zero or more MAC Control Elements (CEs) 23, and optional padding 24. Both the MAC header and the MAC SDUs are of variable sizes. The MAC PDU header consists of one or more MAC PDU sub-headers 25, each sub-header corresponding to either a MAC SDU, a MAC CE, or padding.
A MAC PDU sub-header consists of the six header fields R/R/E/LCID/F/L, except for the last sub-header in the MAC PDU and for fixed sized MAC CEs. The last sub-header in the MAC PDU and sub-headers for fixed sized MAC CEs consist solely of the four header fields R/R/E/LCID. It follows that a MAC PDU sub-header 26 corresponding to padding consists of the four header fields R/R/E/LCID.
FIG. 3A is an illustrative drawing of two versions of a MAC PDU sub-header with six header fields R/R/E/LCID/F/L. The first version 31a includes a 7-bit L field. The second version includes a 15-bit L field.
FIG. 3B is an illustrative drawing of a MAC PDU sub-header 32 with four header fields R/R/E/LCID.
MAC PDU sub-headers have the same order as the corresponding MAC SDUs, MAC CEs, and padding. MAC CEs are always placed before any MAC SDU. Padding occurs at the end of the MAC PDU, except when single-byte or two-byte padding is required but cannot be achieved by padding at the end of the MAC PDU. When single-byte or two-byte padding is required but cannot be achieved by padding at the end of the MAC PDU, one or two MAC PDU sub-headers corresponding to padding are inserted before the first MAC PDU sub-header corresponding to a MAC SDU. If such a sub-header is not present, one or two MAC PDU sub-headers corresponding to padding are inserted before the last MAC PDU sub-header corresponding to a MAC CE. A maximum of one MAC PDU can be transmitted per Transport Block (TB) per UE.
The BSR MAC Control Element reports the amount of data in the buffers for one (short format) or four (long format) Logical Channel Groups (LCGs), i.e., for one Radio Bearer Group (RBG). The short BSR format is 1 byte, and the long format is 3 bytes. Assuming that the MAC header requires only a Logical Channel ID (LCID) and E field, and that the L field can be omitted because the BSR CE has a fixed length, these 6 bits lead to one additional byte in the MAC header and thus the total overhead for each MAC PDU that carries a BSR is 2 bytes or 4 bytes, respectively.
In LTE systems, a Signaling Radio Bearer (SRB) or a Data Radio Bearer (DRB) corresponds to one logical channel (LCH). One or more LCHs are mapped to a LCH group (LCG). LCHs are then configured or associated with priorities. The LCH priority has at least two purposes. For uplink scheduling information, the LCH priority determines whether or not a BSR is triggered. For uplink data transmissions, the LCH priority determines the order in which data is extracted from the UE's buffer and included in the MAC PDU for transmission.
When new data becomes available while the UE has empty buffers, or if this data belongs to a LCH with higher priority than LCHs for which data is already buffered in the UE, a Regular BSR is triggered. Depending of the format used, the BSR reports the state of the UE's buffer for one or more LCGs, i.e., the amount of data available for transmission in each reported LCG.
When a Regular BSR is triggered, a SR is also triggered. If the UE has dedicated resources for SR (D-SR) on the PUCCH, the UE utilizes the D-SR. Otherwise the UE performs a Random Access-based SR (RA-SR). An SR is pending until it is cancelled; an SR can be cancelled, for example, in the Transmission Time Interval (TTI) for which the UE receives a grant, or in the TTI for which the UE has a UL-SCH resource available for a new transmission, or in the TTI for which the UE assembles the MAC PDU for transmission.
When the UE's need to access the system is not known by the network and thus the UE does not have a dedicated preamble, the UE must perform a Contention-Based Random Access (CBRA) to access the system. The CBRA is performed in four steps. First, the terminal transmits a Random Access (RA) preamble; second, the UE receives an RA response (RAR) with an uplink grant; third, the UE makes a first uplink transmission (MSG3); and fourth, contention is resolved only when the UE receives a grant for a new uplink transmission addressed to its Cell-specific Radio Network Temporary Identifier (C-RNTI).